1. Field of the Invention
The invention relates to a medical guide wire generally used as a catheter which is introduced into e.g., a cardiovascular system.
2. Description of the Prior Art
A medical guide wire has been used as a catheter to insert a very thin flexible tube into a blood vessel to take a picture of a vascular system. The medical guide wire has been also used as the catheter to insert a thin wire to place a balloon catheter at a specified blood vessel area so as to positively guide the balloon catheter safely when reopening a clogged coronary artery. These are illustrated by Japanese Provisional Patent Publication No. 4-25024 and Japanese Laid-open Patent Application No.4-292175.
By way of an example, a medical guide wire 1a (referred to as xe2x80x9cguide wirexe2x80x9d hereinafter) advances its front end portion 4a into a complicatedly curved blood vessel 6a or bifurcated blood vessel 7a as shown in FIG. 3. This requires a flexible property for the front end portion 4a, and at the same time, requiring a buckle resistant property against a load resisting in the direction which the guide wire 1a advances. The guide wire 1a is manipulated to turn around its axis at a handle portion 5a outside the body while advancing the front end portion 4a into the blood vessel 7a. This requires two mechanical properties. One is a corresponding torsional rigidity against the turning motion. The other is a favorable maneuverability to advance the front end portion 4a into the blood vessel 7a by the manipulating the handle portion 5a. 
For this reason, the front end portion 4a includes a basic structure having a very thin core front wound by a helical spring.
Before inserting the guide wire 1a into the bifurcated blood vessel 7a, the front end portion 4a is plastically deformed manually into a hook-shaped configuration by way of illustration. When the hook-shaped portion 8a reaches near a bifurcation of the bifurcated blood vessel 7a, the guide wire 1a is turned around its axis to introduce the hook-shaped portion 8a into another path of the bifurcated blood vessel 7a as shown in FIG. 3. This manipulation enables operators to smoothly advance the front end portion 4a into the bifurcated blood vessel 7a. 
In order to make the manipulation easier when introducing the front end portion 4a into the bifurcated blood vessel 7a, it is important to deform the hook-shaped portion 8a easily while ensuring a form-keeping property with the hook-shaped portion 8a. The hook-shaped portion 8a preferably has a property in which the front end portion 4a is easily deformed only in one specified direction, but has a corresponding rigidity in which the front end portion 4a resists to deform in directions other than the specified direction. In order to impart the bending property with the front end portion 4a, the front end portion 4a has been formed rectangular in cross section in which the front end portion 4a has a lateral length and a vertical length.
In the prior guide wire 1a, a priority has been put on the mechanical property that the very thin front end portion 4a is flexible and an outer diameter of the helical spring 3a generally confined to 0.355 mm. This structure is insufficient to impart an appropriate torsional rigidity with the core elongation which serves as a main role when transmitting an inserting force while turning the front end portion 4a around the axis. This may cause the front end portion 4a trapped in e.g., a blood vessel stricture area 9a to block the front end portion 4a from turning around the axis as shown in FIG. 5. This may torsionally deform the core elongation to eventually result in a rupture, which remains a ruptured piece inside the blood vessel.
When the helical spring 3a lacks the appropriate torsional rigidity upon moving in combination with the core elongation, a very thin line element W of the helical spring 3a may plastically deform wavily due to the torsional moment, thereby losing a smoothness of the line element W (FIG. 6) and expanding an outer diameter of the helical spring 3a. This may occur a lesion in a wall of the blood vessel due to the unfavorable deformation of the helical spring 3a engaging directly against the wall of the blood vessel.
When merely strengthening the torsional rigidity of the core elongation and the helical spring 3a in order to solve the above inconvenience, the maneuverability reduces to aggravate pains when inserting the core elongation into the blood vessel to give remedial measures to patients. Upon giving the remedial treatment on the coronary artery stricture area, a medical guide wire has been demanded which concurrently satisfies two conflicting requirements in which the front end portion 4a is sufficiently thin, while at the same time, having a torsional rigidity enough to timely respond to the turning motion transmitted from the handle portion 5a particularly when the front end portion 4a is stuck in the blood vessel stricture area.
Therefore, the present invention has been made with the above drawbacks in mind, it is a main object of the invention to provide a medical guide wire which is capable of satisfying the above two conflicting requirements simultaneously.
According to the present invention, there is provided a medical guide wire comprising: a core member having a flexible front end portion rectangular in cross section; a helical spring wound around the flexible front end portion of the core member; the core member having a torsional rigidity coefficient of 16 or less in terms of K xc3x97103; and the helical spring having a helical diameter-to-line diameter ratio in the range of 2.5xcx9c3.5. Where the torsional rigidity coefficient is a divided value of (a maximum torsional stress based on a torsional moment to which the core member is subjected)/(the torsional moment to which the core member is subjected), and the helical diameter-to-line diameter ratio is a value of (a mean value of outer and inner diameters of the helical spring)/(a line element diameter of the helical spring). In other words, the helical diameter means a distance from a center to a central line of the helical line element of the helical spring.
With the front end portion of the core member placed in the helical spring in which the outer diameter generally measures 0.355 mm, the front end portion is rectangular in cross section in the guide wire according to the invention. Considering the mechanical property that when inserting the core member into the blood vessel, the rigidity against the torsional moment to which the front end portion is subjected varies depending the lateral-to-vertical length ratio of the front end portion, an attention is paid to the torsional rigidity property of the core member. Such is the above structure as to overcome the technical problems i.e., xe2x80x9clack of the insufficient torsional rigidity which the front end portion hasxe2x80x9d and xe2x80x9cunfavorable deformation to which the line element of the helical spring is subjected due to the torsional momentxe2x80x9d. This is done based on experimental test results implemented with a multitude of medical guide wires employed herein.
In the medical guide wire according to the invention, the ratio of the vertical length to the lateral length of the front end of the core member is defined in the range of 1.25xcx9c1.75. This is to ensure a good maneuverability when inserting the front end portion into the blood vessel.